In the field of electric arc welding of the type especially useful in pipe welding, weld puddle temperature and fluidity are controlled by using a variety of welding processes including an arc welding process known as surface tension transfer (STT) welding. This technology was developed and patented by The Lincoln Electric Company, and is disclosed in several patents, including Parks U.S. Pat. No. 4,866,247, Stava U.S. Pat. No. 5,148,001, and Stava U.S. Pat. No. 6,051,810, which are incorporated herein by reference. These patents show STT welding technology in which the present invention is preferably used. Since this technology is well known, incorporation by reference of these patents provides general background information for understanding the preferred implementation of the invention. In practicing STT short circuit welding, a waveform generator produces a precise waveform used in the welding process by creating a series of current pulses the widths of which determine the current flowing in the welding process during specific portions of the cycle. In practicing this type of electric arc welding, as well as other short circuit processes, it is common to use a Power Wave electric arc welder available from the Lincoln Electric Company. Such inverter-based welders are disclosed in Blankenship U.S. Pat. No. 5,278,390 and Hsu U.S. Pat. No. 6,002,104. These patents are also incorporated by reference to disclose the general type of welder used to implement preferred embodiments of the present invention.
In conventional electric arc welding a power source of a constant power, constant voltage, or constant current type delivers electrical power to a weld material arranged in proximity to a weld. The electrical power causes melting of weld material and electrically assisted transfer of the molten weld material across an arc gap to a weld puddle. The weld material is delivered toward the weld via a wire feeder or other arrangement.
In conventional arc welding, the electrical power delivered to the weld is selected to control transfer of weld material to the weld puddle. In short-arc transfer, the electrical power forms a molten drop of weld metal which engages the weld puddle and is then pinched off by high current density. Each drop transfer is caused by a short-circuit that is controlled by a waveform generator. In a short-arc welding process, tens to a few hundred shorts occur per second. In a variant process called pulse welding, detachment of molten drops occur during each of a series of current pulses.
To control the welding process, welding power sources that provide waveform control have been developed. These power sources deliver a series of selectively shaped electrical power waveforms to the weld. The power waveform is optimized for a selected arc welding process, weld metal, wire feed speed, weld joint, and the like. With a suitably tailored waveform, such waveform-controlled power sources improve the speed, consistency, and robustness of the welding process, can substantially improve arc stability and reduce weld metal spatter, and can otherwise optimize the welding process.
In a typical arrangement for waveform-controlled arc welding, a power source providing waveform control interfaces with a user interface computer such as a personal computer, PDA, cell phone, or the like. The computer includes software through which a user can design a selected waveform and communicate the waveform to the power source. The computer further includes control and monitoring software for initiating, controlling, and monitoring the arc welding.
In electric arc welding processes, an important process parameter is the total electrical power or true heat that is input to the weld over the course of a welding process. For conventional arc welding processes, the true heat is suitably characterized by measuring root-mean-square (RMS) voltage and current values and multiplying RMS current by RMS voltage to obtain an RMS true heat value.
A problem arises when using waveform control in that the true heat is not readily measured because the current varies during each of the tailored waveforms. This is especially true between arc conditions and shorted conditions. In particular, in waveform controlled arc welding the product of the RMS current and voltage do not yield an accurate true heat due to phase differences between the current and voltage waveforms which can produce apparent (voltampere) power in addition to real power.
One solution to this problem is to continuously measure the instantaneous current and voltage values and to multiply the measured instantaneous current and voltage values to compute an instantaneous power, which is integrated to determine the true heat. This solution calls for expensive high-speed data communication and data processing hardware. However, data communication and processing hardware used in typical arc welding systems are not fast enough to transfer and process the large volume of instantaneous sampled electrical data produced. Moreover, even if they were fast enough to accomplish true heat monitoring, such process will consume a large portion of the process capacity of the welder""s controller.
The present invention contemplates an improved apparatus and method that overcomes the aforementioned limitations and others.
According to one aspect of the invention, a method is provided for controlling a welding process. A snip of current and voltage samples is measured over a measured portion of the welding process. Based on the snip, a true heat produced during the measured portion of the welding process is computed. The true heat produced during the measured portion is extrapolated over an unmeasured portion of the welding process to produce a true heat value. A welding process parameter is adjusted based on the true heat value.
According to another aspect of the invention, a closed-loop control system is disclosed for a welder that performs a waveform-controlled electric arc welding process. A sampling circuit samples instantaneous current and voltage values of the waveform-controlled electric arc welding process during a snip interval. A true heat processor computes a true heat over the snip interval and extrapolates a true heat over the snip interval plus a delay interval. A true heat setpoint adjustment outputs a selected setpoint true heat value. A controller controls a parameter of the waveform-controlled electric arc welding process based on the extrapolated true heat over the snip interval plus the delay interval and the selected setpoint true heat value.
According to another aspect of the invention, a closed-loop control system is disclosed for a welder that performs a waveform-controlled electric arc welding process. A sampling circuit samples current and voltage values of the waveform-controlled power during a snip measurement interval. A voltage sample thresholding circuit selects voltage sample values corresponding to one of a short condition and an arc condition. A true heat processor computes a true heat over the snip measurement interval by integrating a product of voltage sample values selected by the voltage sample thresholding circuit and corresponding current sample values to estimate a true heat in one of the arc and the short. A controller adjusts the waveform-controlled electric arc welding process to maintain the estimated true heat at a desired value.
According to another aspect of the invention, a method is provided for estimating a true heat of a welding process performed over a welding process interval. A voltage and a current applied by the welding process is sampled over a snip measurement interval that is smaller than the welding process interval. Corresponding sampled voltage and current values are multiplied to generate sampled power values. The sampled power values are integrated over the snip measurement interval to compute a sampled true heat produced during the snip measurement interval. Based on the sampled true heat produced during the snip measurement interval, the true heat of the welding process produced during the welding process interval is estimated, preferably by extrapolation.
According to yet another aspect of the invention, an arc welding apparatus is disclosed for performing a waveform-controlled arc welding process. An arc welding torch interacts with an associated weld. A power supply applies a selected waveform-controlled power to the weld via the arc welding torch over a process interval. The power supply includes a sampling circuit for sampling instantaneous current and voltage values of the power during a snip portion of the process interval. A processor is designed to (i) compute a true heat over the snip portion of the process interval, and (ii) extrapolate a true heat over the process interval based thereon.
According to still yet another aspect of the invention, a method is provided for estimating a true heat of a welding process performed over a welding process interval. A voltage and a current applied to the weld are sampled over a snip interval that spans at least a portion of the welding process interval. Corresponding sampled voltage and current values are multiplied to generate sampled power values. Each sampled power value is classified as one of a sampled short power value and a sampled arc power value. A true heat in the short over the welding process interval is computed based on the sampled short power values. A true heat in the arc over the welding process interval is computed based on the sampled arc power values.
Numerous advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiment.